1. Field of the Invention
The present invention relates to an apparatus for measuring a magnetic field, and particularly to an apparatus for eliminating, in measuring a very weak magnetic field, external magnetic noise components generated by other magnetic sources and mixed in with the very weak magnetic field to be measured.
2. Description of the Related Art
In recent years, a very weak magnetic field produced by a human body, etc., is measured by a highly sensitive apparatus employing a SQUID (superconducting quantum interference device).
In measuring the very weak magnetic field, external magnetic noise causes a problem. Sources of the external magnetic noise are geomagnetism, electric cars, elevators, electric appliances such as computers, etc. These sources are located very far, compared to a distance between a pickup coil of the measuring apparatus and a source of the very weak magnetic field to be measured. The external magnetic noise may therefore be considered as a uniform magnetic field or uniform gradient field whose direction do not change around the source of the very weak magnetic field.
Based on this assumption, the external magnetic noise can be removed by employing, as the pickup coil, a gradiometer such as a first-order gradiometer and a second-order gradiometer. This type of pickup coil has, however, manufacturing errors which prevent a complete removal of the noise.